Objective: Any possible change on the quality of voice of the patient after rapid maxillary expansion (RME) treatment should be clarified at pre-treatment counselling of patients and guardians. The aim of this study is to assess the impact of rapid maxillary expansion on spelling of eight Turkish vowels.

Materials and method: Six patients whose treatment plan was approved as RME and going to wear acrylic cap type hyrax were recruited for the study. The recordings of eight Turkish vowels (/a/, /ɛ/, /ɯ/, /i/, /ɔ/, /oe/, /u/, /y/) were pronounced one by one by the patients. Acoustical analysis was performed using PRAAT analysis tools. Fundamental frequencies (F0), formant frequencies of F1, F2, F3, F4 and vowel durations for the Turkish vowels were measured before (T0) and after (T1) the RME.

Results: A significant difference in the mean F2 /i/ and F3 /i/ (p=0.001 and p=0.002, respectively) and F3 /ɯ/ was found at times T0 and T1 (p=0.022). There were no statistically significant difference between the changes in F0 values for both gender and vowel durations at T0 and T1 (p>0.05). There were no statistically significant difference in any of the other formants F1, F2, F3, and F4 for the vowels /a/, /ɛ/, /ɔ/, /oe/, /u/ and /y/.

Conclusion: Subject to the small sample size limitation of this study, spelling of vowels of /i/ and /ɯ/ are lowered after maxillary expansion. The possibility of voice change after RME should be informed to the patient before treatment.